I read the article in PC Gamer and it didnt say anything about more character slots and none of the dev posts in the other article have said anything about it.
I believe Gaile Gray mentioned she hadn't heard anything, but would check it out. It might be somthing not decided yet. There is still months to go before release.
@Covah - you don't need to own Prophecies, just any one of the chapters. The earthquakes will open entrances to the tunnels in Cantha and Elona also. The gameplay will be in the tunnels and then in areas outside current Tyrian map.
Well not that might not be true. What makes me think there wont be is that it is a expansion, not a stand-alone game like the others. I really hope it does, but I have doubts.